ISBN 9783843905350

Mike GyamfiScanning Tunneling Microscopy and Spectroscopy of Adatoms on Graphene

135 Seiten, Dissertation Universität Hamburg (2012), Softcover, A5

Zusammenfassung / Abstract

In this thesis the properties of epitaxial graphene as well as Ni, Fe, and Co adatoms were investigated by means of scanning tunneling microscopy and spectroscopy at low temperatures. The experiments were performed on three different substrates leading to different properties of monolayer graphene. While the Dirac point is not preserved in case of Ru(0001), graphene is n and p doped in case of SiC(0001) and Pt(111), respectively.

Firstly, two new experimental setups were constructed. Each of them combines a home-made scanning tunneling microscope with all required surface science techniques for in situ tip and sample preparation and characterization. One setup allows measurements at variable temperatures between 20 K and 300 K while the other one is operated at 4.8 K. The key features of the latter microscope are a fully rotatable external magnetic field and adatom deposition below 15 K.

Secondly, monolayer graphene grown on the silicon terminated SiC(0001) surface was investigated. After a precharacterization of the samples, the magnetic transition metal atoms nickel and cobalt were deposited and investigated. In case of Ni monomers a diffusion barrier of 240 meV and an orbital selective coupling of atomic states and graphene's Dirac electrons was revealed. In case of Co monomers the experimental results did not indicate a Kondo resonance contrary to theoretical predictions.

Thirdly, graphene on Ru(0001) was in situ grown and studied. Monolayer graphene is characterized by a moiré pattern and a strong interaction. The lateral inhomogeneity of the moiré pattern was revealed by means of a bias dependent corrugation, field emission resonances, Ru bulk states, and adatom deposition. Additionally, defects in and Fe adatoms on bilayer graphene were investigated.

Fourthly, monolayer graphene on Pt(111) was in situ prepared and analyzed. It represents quasi-free-standing graphene due to a weak interaction. Ni, Fe, and Co monomers were deposited at low temperatures and found to exhibit a bistability. Their apparent heights as well as their electronic structures switch between two well defined states. The room temperature deposition of Co results in the intercalation of graphene by a Co monolayer. Contrary to the weak interaction between graphene and Pt(111), a strong interaction between graphene and the intercalated Co was observed.